This invention relates generally to electrical apparatus, and more particularly to a gas-insulated epoxy bushing which has a ground shield cast into the base of the bushing wall.
Air-to-gas bushings have typically been utilized on conventional gas-insulated equipment such as circuit breakers or compressed gas-insulated transmission lines. Such conventional air-to-gas bushings typically comprise an outer insulating shell of either procelain or an epoxy material, with a centrally disposed conductor extending down through the center of the housing. Stress control shields are utilized to control stresses along the housing, and generally are internal, concentric throat shields disposed within the hollow shell, and occasionally external grounded shields are also utilized. The interior of the housing is filled with an insulating gas, typical of which is sulfur hexafluoride gas at a pressure of 50 lb./sq. in. gauge.
The operation of the typical procelain gas bushing has been satisfactory, but there are some disadvantages associated with it. The major disadvantage is that in the event of a puncture, the procelain shatters. The usual failure location is associated with the top of the internally disposed throat shield. The electric field here on the side facing the air is high, and as a consequence there are often intense corona discharges along the procelain surface in the air. Occasionally, flashover occurs along the surface of the procelain in the air, then punctures through the procelain shell to the top of the internal throat shield.
In the design of the bushing, it is necessary to have gas clearance between the conductor and the throat shield, and the dimensions are such as to prevent a breakdown in the gas. In addition, clearance is required between the edge of the throat shield and the inside surface of the procelain, so that there are no discharges from the top of the throat shield to the procelain.
Designs utilizing an epoxy shell instead of the procelain shell generally utilize the same type of stress control means as heretofore described. Additionally, an external toroid at the top of the bushing is occasionally utilized. In the event of a flashover failure, there is still a tendency for the breakdown channel to go from the air side through the epoxy to the top of the internal throat shield. Even if the epoxy is formulated so as not to shatter, there is still a puncture through the epoxy for the insulating gas to leak out.